double_dispatch.java

class Handler {
	public void handle(InputEvent ie) {
		ie.dispatch(this);
	}
	
	public void handleInput(InputEvent ie) {
		System.out.println("ERROR - NO HANDLER DEFINED");
	}
	
	public void handleInput(MouseEvent me) {
		System.out.println("MOUSE EVENT!");
	}
	
	public void handleInput(KeyboardEvent ke) {
		System.out.println("KEYBOARD EVENT!");
	}
}

class InputEvent {
	public void dispatch(Handler h) { h.handleInput(this); }
}

class MouseEvent extends InputEvent {
	public void dispatch(Handler h) { h.handleInput(this); }
}

class KeyboardEvent extends InputEvent {
	public void dispatch(Handler h) { h.handleInput(this); }
}

class FooEvent extends InputEvent {
	// no explicit handler defined
	public void dispatch(Handler h) { h.handleInput(this); }
}

class Main {
	public static void main(String[] args) {
		Handler h = new Handler();
		h.handle(new MouseEvent());
		h.handle(new KeyboardEvent());
		h.handle(new FooEvent());
	}
}